The objective of this research program is to develop and refine realistic predictive models for particle deposition efficiencies and patterns in the human tracheobronchial tree. The models will cover respirable particles of all sizes, a wide range of constant and cyclic respiratory flows, and be applicable to population subgroups as well as averages. They will be based upon available data from studies on humans in-vivo, and available data and new data on the distribution of deposition in hollow bronchial casts. The new studies will be performed using gamma-tagged monodisperse aerosols, twelve new bronchial casts which extend to 2 mm airways, new replicate large airway casts, new multiple lobule casts of airways less than or equal to 2 mm, and appropriate collimated scintillation detectors. A variable orifice larynx model will be construction for the deposition studies with cyclic flows. High resolution surface density measurements will be made by counting the numbers of particles on bifurcations and other selected cast surfaces with a scanning electron microscope. These measurements will establish concentration factors and identify locations for the cells with the highest exposures to inhaled particles; thereby providing a basis for the deposition components of an airway surface dosimetry model related to human bronchial cancer risk.